# What is the total mass of an O atom?

## Homework Statement

It's a simple problem. What is the total mass of an oxygen atom?

## Homework Equations

Mass of electrons, protons, and neutrons, as well as their respective amount in an O atom

## The Attempt at a Solution

Knowing the number of e, p, and n in an oxygen atom, I can calculate the total mass of O easily. But I am just unsure if there is a more accurate way of doing this. My doubt is based on that I remember there is this mass defect thing in the nucleus which makes the total mass of a nucleus being not exactly equal to the sum of the masses of protons and neutrons it is composed of.

Drakkith
Staff Emeritus
My doubt is based on that I remember there is this mass defect thing in the nucleus which makes the total mass of a nucleus being not exactly equal to the sum of the masses of protons and neutrons it is composed of.
That's right. The mass of an atom is not exactly equal to the sum of the masses of the individual particles composing the atom. This is because letting these particles bind together releases energy and, by extension, mass. (Look up binding energy) However, if you're still in moderately basic physics/chemistry class then this small difference doesn't matter and you can usually just use the values given in your book for each particle.

I can actually find the data of atomic weight for every element, but are the electrons included in these numbers or are they just the nucleus mass? The mass of e differs from that of p by about 4 orders of magnitude, but the data usually has more than 4 figures behind decimal so I think electron mass can be important here.

DrClaude
Mentor
I can actually find the data of atomic weight for every element, but are the electrons included in these numbers or are they just the nucleus mass?
It includes the electrons. But you have to be careful: if there is a single number for each element, it means that it is the average atomic weight taking into account natural isotopic abundance.

The following resource from NIST has the atomic weight of individual isotopes for all naturally occurring isotopes: http://physics.nist.gov/PhysRefData/Handbook/periodictable.htm

• maNoFchangE
There are several numbers which might be confused as the real atomic mass. For example, the data for nitrogen displays an atomic weight of 14.00674 (centered on the page), but in the table there is also 14.003074 for the isotope 14N. Is the former the average weight?
Another source like this specify a slightly different value
14.00643, judging from its closeness to the first value in NIST's website, can this value possibly be the average mass?

DrClaude
Mentor
There are several numbers which might be confused as the real atomic mass. For example, the data for nitrogen displays an atomic weight of 14.00674 (centered on the page), but in the table there is also 14.003074 for the isotope 14N. Is the former the average weight?
Another source like this specify a slightly different value
14.00643, judging from its closeness to the first value in NIST's website, can this value possibly be the average mass?
Check for yourself: 14.003074 × 99.63% + 15.000108 × 0.37% = 14.000676

Note that you should always use the value for the atomic weight given, not the one calculated from individual masses and abundances.

not the one calculated from individual masses
You mean the individual masses of e, p, and n? Is it because if I do that I neglect the binding energy of the nucleus?

DrClaude
Mentor
You mean the individual masses of e, p, and n? Is it because if I do that I neglect the binding energy of the nucleus?
No, I meant calculated from the massses of individual isotopes. In the example for N, you see that the value I calculated is not the same as the stated atomic weight. Use the latter if you need the atomic weight instead of the mass of an isotope.

To clarify things: the atomic weight is there if you need to calculate something for a representative sample, like one liter of N2, which will contain a mixture of isotopes. For calculations involving a sinlge atom (or molecule), use the isotope's mass.